Biomedical signal acquisition has gained much attention in recent years due to the fast growing market for portable biomedical electronics such as wearable or implantable health monitoring devices. Such devices typically include an analog front-end for signal amplification and conditioning, and an analog-to-digital converter (ADC) for quantization. Additionally, these devices often demand multi-channel operation to record biological signals from various sites.
A direct method of implementing a multi-channel signal acquisition system is to employ an independent analog front-end and ADC for each channel. However, this method is cost-inefficient as it requires multiple ADCs which require additional area to implement.
Therefore, a multi-channel signal acquisition system is conventionally implemented by utilizing an analog multiplexer. FIG. 1 shows a typical conventional multi-channel signal acquisition system 100. In FIG. 1, only two channels are shown to illustrate the concept; however, it would be appreciated that an m number of channels can be used to form an m-channel signal acquisition system. The system 100 includes multiple analog front-ends 102 to acquire and amplify signals from different sites. By utilizing an analog multiplexer 104, the amplified signals are multiplexed to an ADC 108 for quantization. Quantization for each channel is performed one after another in a sequential order.
Although the conventional structure as shown in FIG. 1 can reduce the number of ADCs required in a multi-channel signal acquisition system, the ADC has a very limited time to sample an input signal during the sampling phase. As a result, the multiplexer 104 needs a preceding buffer 110 along with a following buffer 106, both with an exceedingly high bandwidth, as compared to the bandwidth of the input signal, to minimize quantization error due to sampling error. As a higher bandwidth requires a larger biasing current, a high bandwidth buffer is unfavorable in a system optimized for e.g. low power and high energy efficiency. For example, in one conventional approach, the power dissipation for the buffer can be more than 30 times the power of a low-noise preamplifier. Furthermore, incorporating an analog multiplexer in a multi-channel signal acquisition system is equivalent to inserting additional switches in the critical signal path and producing undesirable signal distortion, especially in a low-voltage operation with limited voltage headroom. Lastly, channel crosstalk is also a common issue in an analog multiplexing system.
FIG. 2 shows an ADC conversion timing diagram 200 for the multi-channel signal acquisition system 100 shown in FIG. 1. Assuming that a successive approximation (SA) ADC is used for quantization and the quantization is performed under an ADC clock 202, every channel requires at least n+1 clock cycles (Tclk) 210 for an n-bit quantization. In this case, an n-bit conversion 204 takes a period of nTclk 211 while a sampling phase for each channel 206, 208 is limited to a period of Tclk 212. Because of such a short sampling time, the preceding buffer (e.g. 110 in FIG. 1) requires a very large bandwidth compared to the bandwidth of the target signal, as described above, as well as a high slew rate. This may lead to low system power efficiency. The bandwidth and slew rate requirements of the buffer may be reduced, but at the cost of a higher ADC conversion rate and a faster ADC clock, which may also lead low system power efficiency in return.
Therefore, a need exists to provide a multi-channel signal acquisition system that seeks to address at least some of the above problems.